It's probably not a small amount. But not only that - you need to keep in mind that it also requires some time from the people working on the project.

Do these conferences really bring in some instant benefits? I've always considered them as a tool for networking mostly... but that takes some time.

Well, there won't be anybody waiting with a bag of money if that's what you are asking

but some serious deals have been negotiated on those conferences and you can't neglect the importance of networking.

That's somehow i see it. No one guarantees that you will be enriched if you participate in one of the conferences, but given the number of participants and visitors and the media coverage project can get a lot from it.